This invention relates to control of the energization of gas discharge lamps, and more particularly relates to a novel pulse network for the ballast of a fluorescent lamp which is to be dimmed by a phase control unit.
It is well known that fluorescent lamps can be dimmed through the use of thyristor-type phase control units in series with an inductive ballast and the lamp. The terms thyristor phase control or thyristor switching will be used hereinafter to broadly designate the well known phase control function which can be obtained by any suitable switching device such as a thyristor, triac, transistor, break-over diode, or the like. When the ballast is a simple series inductor, phase control can produce dimming of the lamp output to about 20% of the available light output of the lamp. However, when dimming below 20% , severe lamp flicker and uneven light output between parallel lamps will occur.
The performance of such dimming circuits and lamps can be improved by increasing the supply voltage and the ballasting inductance. By increasing supply voltage, there will be a more continuous flow of energy to the lamp arc during the off period of the thyristor or other switch device to prevent lamp deionization during the off period.
It is also known to employ a pulse forming circuit connected to a tap on the ballast inductance. Thus, a series connected resistor and capacitor forming the pulse network are connected to a tap on a series ballast. During the beginning of the conduction within each phase controlled cycle, the pulse network will appear as a low impedance, and will cause transformer action between the tapped sections of the inductor. Thus, a high voltage can be applied to the lamp to ensure its adequate striking under substantial phase controlled conditions. As the capacitor of the series capacitance and resistive circuit charges, the transformer action reduces and the main power source is eventually applied to the lamp through the series inductance circuit. This arrangement has been found to produce positive ionization of the plasma within the lamp each half cycle and provides repeatable lamp conduction characteristics from cycle to cycle so long as the a-c supply voltage is high enough, and so long as the frequency is relatively high (greater than about 50 Hz.). When the frequency is low, for example 50 Hz., the lamp or parallel lamps tend not to reionize completely, particularly at the low end of the dimming range. This gives rise to lamp flicker and poor matching of light output between lamps when dimming below 10% of full light output.
To overcome this problem, it is a common expedient to impress an incandescent lamp load directly in parallel with the tapped portion of the ballast inductor and the pulse network. The incandescent lamp load is about 10 watts for each fluorescent lamp which is in the entire system driven from a common phase control assembly. When an incandescent lamp load of this size is applied across all parallel ballasts of a system, the pulse network can produce excellent dimming operation to below 1% of the available light output of the lamps with no significant lamp flicker. Thus, in an installation employing 20 lamps with 20 respective ballasts, for example, a 200 watt incandescent lamp, or an equivalent resistive load, is employed for the best dimming performance.
The use of incandescent lamps in addition to the pulse network wastes power. Moreover, the additional resistive load is commonly added by the equipment installer, rather than the ballast manufacturer, so that the design and connection of the load is uncontrolled, and produces an additional maintenance problem. Moreover, when ballasts and lamps are added or removed from the system, the incandescent lamp load must be changed for optimum dimming performance.
In some cases, dimmer manufacturers have included such a resistive load directly in the dimmer phase-control circuit housing. This, however, substantially increases the size of the housing because of the need for dissipating the heat approximately 10 watts for each fluorescent lamp which may be connected to the controller.